US 7038234 B2
A super-lattice thermoelectric device. The device includes p-legs and n-legs, each leg having a large number of alternating layers of two materials with differing electron band gaps. The n-legs in the device are comprised of alternating layers of silicon and silicon germanium. The p-legs includes alternating layers of B4C and B9C. In preferred embodiments the layers are about 100 angstroms thick. Applicants have fabricated and tested a first Si/SiGe (n-leg) and B4C/B9C (p-leg) quantum well thermocouple. Each leg was only 11 microns thick on a 5 micron Si substrate. Nevertheless, in actual tests the thermocouple operated with an amazing efficiency of 14 percent with a Th of 250 degrees C. Thermoelectric modules made according to the present invention are useful for both cooling applications as well as electric power generation. This preferred embodiment is a thermoelectric 10×10 egg crate type module about 6 cm×6 cm×0.76 cm designed to produce 70 Watts with a temperature difference of 300 degrees C with a module efficiency of about 30 percent.
1. A thermoelectric module comprised of:
A) a plurality of n-legs comprised of very thin alternating layers of silicon and silicon germanium; and
B) a plurality of p-legs comprised of very thin alternating layers of boron carbide comprise two different stoichiometric forms of boron carbide;
said p-legs and said n-legs being electrically connected to produce said thermoelectric module.
2. A thermoelectric module as in
3. A thermoelectric module as in
4. A thermoelectric module as in
5. A thermoelectric module as in
6. A thermoelectric module as in
7. A thermoelectric element as in
8. A thermoelectric element as in
9. A thermoelectric element as in
10. A thermoelectric element as in
11. A thermoelectric element as in
12. A thermoelectric element as in
13. A method of producing thermoelectric modules comprising the steps of:
A) depositing on a thin substrate a very large number of very thin alternating layers of silicon and silicon germanium to form a Si/SiGe thermoelectric film;
B) forming a stack of said Si/SiGe films produced as in step A),
C) producing a plurality of Si/SiGe thermoelectric n-legs from said stacks of Si/SiGe films,
D) depositing on a thin substrate a very large number of very thin alternating layers of B4C and B9C to form a B4C/B9C thermoelectric film;
E) forming a stack of said B4C/B9C films produced as in step A),
F) producing a plurality of B4C/B9C thermoelectric p-legs from said stacks of B4C/B9C films,
G) forming a thermoelectric module from said plurality of Si/SiGe thermoelectric n-legs and said plurality of B4C/B9C thermoelectric legs,
said p-legs and said n-legs being electrically connected to produce said thermoelectric module.
This invention was made in the course of contracts with agencies of the United States government and the government has rights in this invention. This Application is a continuation in part of U.S. patent application Ser. No. 10/021,097, filed Dec. 12, 2001, now U.S. Pat. No. 6,828,579 and also claims the benefit of Provisional Application Serial No. 60/460,057 filed Apr. 3, 2003. The present invention relates to thermoelectric devices and in particular to very thin lattice thermoelectric devices.
Workers in the thermoelectric industry have been attempting too improve performance of thermoelectric devices for the past 20–30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electrical conductivity. Experiments with super-lattice quantum well materials have been underway for several years. These materials were discussed in an paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in super-lattices. These super-lattices contain alternating conducting and barrier layers and create quantum wells that improve electrical conductivity. These super-lattice quantum well materials are crystals grown by depositing semiconductors in layers whose thicknesses is in the range of a few to up to about 100 angstroms. Thus, each layer is only a few atoms thick. (These quantum well materials are also discussed in articles by Hicks, et al and Harman published in Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. The articles project theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials might provide very great increases in electric conductivity without adversely affecting Seebeck coefficient or the thermal conductivity. Harmon of Lincoln Labs, operated by MIT has claimed to have produced a super-lattice of layers of (Bi,Sb) and Pb(Te,Se). He claims that his preliminary measurements suggest ZTs of 3 to 4.
The present inventors have demonstrated that high ZT values can definitely be achieved with Si/Si0.8Ge0.2 super-lattice quantum well. (See, for example, U.S. Pat. No. 5,550,387.) The present inventors have had issued to them United States patents in 1995 and 1996 which disclose such materials and explain how to make them. These patents (which are hereby incorporated by reference herein) are U.S. Pat. Nos. 5,436,467, 5,550,387.
U.S. Pat. Nos. 6,096,964 and 6,096,965 disclose techniques for making thermoelectric elements by depositing thin alternating layers of semi-conductor material and barrier layers on very thin substrates to create quantum wells. All of the above-cited patents are incorporated herein by reference.
What are needed are improved thermoelectric devices that can be produced efficiently for practical application.
The present invention provides a super-lattice thermoelectric device. The device is comprised of p-legs and n-legs, each leg being comprised of a large number of alternating layers of two materials with differing electron band gaps. The n-legs in the device are comprised of alternating layers of silicon and silicon germanium. The p-legs are comprised of alternating layers of B4C and B9C. In preferred embodiments the layers are about 100 angstroms thick. Applicants have fabricated and tested a first Si/SiGe (n-leg) and B4C/B9C (p-leg) quantum well thermocouple. Each leg was only 11 microns thick on a 5 micron Si substrate. Nevertheless, in actual tests operated the couple operated with an amazing efficiency of 14 percent with a Th of 250 degrees C. Thermoelectric modules made according to the present invention are useful for both cooling applications as well as electric power generation. This preferred embodiment is a thermoelectric 10×10 egg crate type module about 6 cm×6 cm×0.76 cm designed to produce 70 Watts with a temperature difference of 300 degrees C. with a module efficiency of about 30 percent. The module has 98 active thermoelectric legs, with each leg having more than 3 million super-lattice layers. The n-legs are alternating layers of Si/SiGe and the p-layers are alternating layers of B4C/B9C.
Applicants have produced a test quantum well thermoelectric couple with 11 microns of thermoelectric layers on a 5-micron silicon film that has operated at 14 percent conversion efficiency. This efficiency was calculated by dividing the power out of the couple by the power in to an electric heater with no correction for extraneous heat losses. The accuracy of the experimental set-up used was validated by measurement of the 5 percent efficiency of a couple fabricated of bulk Bi2Te3 alloys. The test set-up is shown in
The thermodynamic efficiency, η, for a thermoelectric power generator is given by:
In this first preferred embodiment thermoelectric elements are made with p-type legs comprised of super-lattices of alternating layers of B4C and B9C and n-type legs comprised of a super-lattices of alternating layers of Si and SiGe. The B4C/B9C legs (as a p-legs) functions as thermoelectric elements without added doping. The SiGe is n-doped at 1018 to 1020/mole.
The following special materials and equipment are needed to produce thermoelectric film for this first preferred embodiment:
To produce Si/SiGe legs for the thermoelectric module for this embodiment, stacks of 116 films are required. So the process described above is repeated until the required quantity of film is produced. The film is then stacked as indicated in
B4C/B9C legs for this thermoelectric module are produced with the same procedure described above except B4C replaces the silicon as target material and as thermoelectric layers. The substrate however is silicon and the 0.05 micron silicon layer between sequences of layers is also silicon, all as shown in
The 98 legs prepared as described above for this embodiment should now be loaded into thermoelectric egg crates of the type well known in the thermoelectric art. One such egg crate is shown at 38 in
The completed module has dimensions of about 6 cm×6 cm×1.4 cm and is designed to produce 70 Watts at a temperature difference of 300 degrees C. with a heat flux of 10 W/cm2. The modules are useful for waste heat recovery, auxiliary power units, self-powered engine heaters, space power, and low temperature detector cooling. Applicants estimate high volume cost of these modules at $0.20/Watt to $0.50/Watt compared to about $1/Watt for conventional bulk thermoelectric modules.
As described in U.S. Pat. Nos. '467, '387, '964 and '965, quantum well thermoelectric material is preferably deposited in layers on substrates. For a typical substrate as described in those patents, heat loss through the substrate can greatly reduce the efficiency of a thermoelectric device made from the material. If the substrate is removed some of the thermoelectric layers could be damaged and even if not damaged the process of removal of the substrate could significantly increase the cost of fabrication of the devices. The present invention provides a substrate that can be retained. The substrate preferably should be very thin with a low thermal and electrical conductivity with good thermal stability and strong and flexible.
Silicon is the preferred substrate material for depositing the Si/SiGe and B4C/B9C layers. Si is available commercially in films as thin as 5 microns from suppliers such as Virginia Simi-conductor with offices in Fredricksburg, Va. By using a 5-micron substrate the amount of bypass heat loss can be held to a minimum. For commercial applications the quantum well film will be approximately 35 microns thick as explained above. Thus the ratio of quantum well thickness to substrate thickness is more than sufficient to greatly minimize by-pass heat losses. The silicon film is stable at much higher temperatures than Kapton.
Kapton is a Product of DuPont Corporation. According to DuPont Bulletins:
Kapton® polyimide film possesses a unique combination of properties that make it ideal for a variety of applications in many different industries. The ability of Kapton® to maintained its excellent physical, electrical, and mechanical properties over a wide temperature range has opened new design and application areas to plastic films.
Kapton® is synthesized by polymerizing an aromatic dianhydride and an aromatic diamine. It has excellent chemical resistance; there are no known organic solvents for the film. Kapton® does not melt or burn as it has the highest UL-94 flammability rating: V-0. The outstanding properties of Kapton® permit it to be used at both high and low temperature extremes where other organic polymeric materials would not be functional.
Adhesives are available for bonding Kapton® to itself and to metals, various paper types, and other films.
Kapton® polyimide film can be used in a variety of electrical and electronic insulation applications: wire and cable tapes, formed coil insulation, substrates for flexible printed circuits, motor slot liners, magnet wired insulation, transformer and capacitor insulation, magnetic and pressure-sensitive tapes, and tubing. Many of these applications are based on the excellent balance of electrical, thermal, mechanical, physical, and chemical properties of Kapton® over a wide range of temperatures. It is this combination of useful properties at temperature extremes that makes Kapton® a unique industrial material.
Applicants have demonstrated that Kapton can be useful as a substrate film for super-lattice thermoelectric layers when high temperature use is not planned. Applicants have shown that a crystal layer laid down between the Kapton® substrate and the series of very thin conducting and barrier layers greatly improve thermoelectric performance especially for n-type layers. The preferred technique is to lay it on about 1000 Å thick in an amorphous form then to crystallize it by heating the substrate and the silicon layer to about 350° C. to 375° C. When Kapton® is used as a substrate it can be mounted on a crystalline base that can be sand blasted off the Kapton® after the thermoelectric film is deposited.
Many other organic materials such as Mylar, polyethylene, and polyamide, polyamide-imides and polyimide compounds could be used as substrates. Other potential substrate materials are oxide films such as SiO2, Al2O3 and TiO2. Mica could also be used for substrate. As stated above, the substrate preferably should be very thin a very good thermal and electrical insulator with good thermal stability, strong and flexible.
While the above description contains many specificites, the reader should not construe these as limitations on the scope of the invention, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision many other possible variations within its scope. The preferred layer thickness is about 100 Angstroms; however, layer thickness could be somewhat larger or smaller such as within the range of 200 Angstroms down to 10 Angstroms. It is not necessary that the layers be grown on film. For example, they could be grown on thicker substrates that are later removed. There are many other ways to make the connections between the legs other than the methods discussed. Accordingly, the reader is requested to determine the scope of the invention by the appended claims and their legal equivalents, and not by the examples which have been given.